(1) Field of the Invention
The present invention relates to a method and apparatus for submersion and positioning of large, heavy elements on a preselected site on the sea bottom.
The invention is particularly attractive in connection with large sea depths for elements having a weight of 1000-5000 tons or more which are to be installed with great accuracy in offshore areas with rough weather, in some cases in close vicinity of already-installed platforms or other equipment which easily may be damaged in case of a collision with such a heavy element during the submersion and installation of the element.
In the following specification, the elements which shall be submerged and positioned on the sea bottom are generally designated as "sink elements".
(2) State of the Prior Art
It is known to use large, heavy sink elements as permanent anchoring for tension leg platforms, bridge foundations or the like. In some cases such sink elements have a substantial weight. If cranes are utilized in connection with the immersion of the element, large demands must be be met in regard to crane capacity. The crane capacity, in addition to the weight of the sink element, also must be sufficient to sustain supplementary forces which will be created due to the movements of the crane vessel caused by winds, currents, etc., which movements inevitably will be transferred to the sink element via the crane cables. At larger depths, the weight of the cables themselves will inherently set a limit. This problem is sought to be avoided by using light weight cables made of new materials. Furthermore, the natural frequency of the system will increase from small and negligible values to values which are comparable or identical with the frequencies or periods for sea waves and/or with the natural frequency of the crane vessel.
It is also known to provide sink elements with positive buoyancy such that the immersion weight or force is being reduced or eliminated. The buoyancy is usually provided by designing the sink element with buoyancy chambers which may be filled with water or other ballast materials when the installation has been finalized.
It is furthermore known to connect sink elements with separate buoyant bodies or floats which, in connection with a controlled ballasting/deballasting procedure, are utilized during the immersion of the sink element, without the use of cranes, until the sink element is positioned at a short distance or height above the sea bottom and thereafter displace the sink element laterally until the preselected installation site has been reached. This preselected height above the sea bottom is defined with reference to the sea surface either on the basis of a crane or crane vessel (if such is used), or by means of a floating ballastable buoy, and where the last part of the immersion operation down to the sea bottom takes place by a controlled ballasting of the sink element "floating element", which is connected to the buoy with a cable having a preselected length. The buoy has a small water plane area, and has height along the water surface which exceeds the remaining height between the sink element and the sea bottom. This immersion procedure is shown in Applicant's U.S. Pat. No. 4,909,671. In Norwegian Patent application Ser. No. 88.2948 is shown in similar buoy utilized for identification of the position of the sink element during the immersion operation.
Regardless of which immersion system is being utilized, the movements and the position of the sink element, both vertically and horizontally, are being controlled from the sea surface.
Adequate control in the vertical direction is obtained by the known conventional system, and undesirable movements are eliminated. Thus, short amplitude movements induced by waves are eliminated by the insensibility of the system due to long natural frequencies, and the long amplitude movements which are induced by tide water is eliminated by adjusting the length of the cable (if the crane is utilized), or by means of ballasting/deballasting if a surface buoy is being utilized.
Undesirable movements in the horizontal plate are, however, induced by:
a) crane movements and hydrodynamic forces created by currents and waves, particularly in shallow waters, if a crane is being utilized, and
b) hydrodynamic forces, substantially created by currents, if the sink element is installed utilizing either the buoyancy of the sink element itself or by means of supplied buoyancy.
These movements are sought to be counteracted from the sea surface in an "active" fashion by means of tension control of the control cables from an auxiliary vessel. If the applied tension from the vessel accidentally should fail (by cable failure, engine troubles, or anchor loosening), the control is lost, and the sink element initiates an uncontrolled movement. Unintended movements may also occur due to erroneous maneuvering of the auxiliary vessel. In order to limit the consequences of such incidents, it may be important to arrange special apparatus in order to protect existing installations on the sea bottom. Such apparatus may involve large expenses, since large forces are involved, and these forces may also easily cause damage to the sink element which, as a safe guard, must be protected in several ways.